The invention relates to a high-frequency device to generate a plasma arc for the treatment of biological tissue according to the precharacterizing clause of claim 1.
High-frequency devices of this kind have been disclosed in the patents DE-OS 37 10 489, EP 0 353 178 A2 and WO 93/01758. The surfaces of human tissues can be treated in a medium consisting of varous gases. Often the spatial configuration of the plasma beam can be improved by directing towards the tissue a concentrated beam of a gas, the ionization field strength of which is lower than that of the gas in the surrounding area.
Regarding the supply of the high-frequency energy that is needed to generate a plasma beam, in all cases there are two fundamentally problematic considerations. First, it is necessary to ignite the plasma beam reliably after it has become extinguished during use and when it is initially turned on. Second, the HF current intensity in the ignited plasma beam should be adjustable so that the intensity is suitable for the particular medical application.
The latter problem is extensively evaluated in DE-OS 37 10 489 and in EP 0 353 178 A2, and is solved by an extremely complicated device for the regulation and control of the energy supplied to the plasma. The controlled system here consists of a HF driver circuit that charges an output oscillator with a predetermined frequency, such that the output oscillator discharges at its resonant frequency so as to deliver electrical energy to the tissue. By means of frequency dividers, frequency signals are obtained with which to control the application and the duration of the feeder or driver pulses sent to the output oscillator. The HF driver circuit thus has a switching function within a complicated control circuit. In order for such a device to produce reliable ignition with an electrode voltage substantially above the ignition voltage of the plasma beam, it is essential to adjust the driver pulses so that they last for a relatively long time. After ignition of the plasma by the control system, the intensity must be adjusted to a level suitable for the medical application in a short time, which is predetermined by the time constant of the control system.
The objective of the invention is therefore to improve the high-frequency device to generate a plasma arc according to the precharacterizing clause of claim 1 by means of a simple circuit.
This objective is achieved by a high-frequency device with the characteristics of claim 1.
With the present circuit, when the plasma is in the non-ignited state, the high-frequency voltage at the electrode is appreciably above the level needed to ignite the plasma arc, and when the plasma is in the ignited state, the high-frequency current within the plasma has the intensity suitable for the specific medical application; similarly, if the plasma arc becomes extinguished during use, the high-frequency voltage at the electrode again rises to a level above that needed to ignite the plasma arc, so that re-ignition occurs automatically.
A substantial advantage of the solution in accordance with the invention is that without the provision of additional control circuitry, and hence with no elaborate technical additions, a stable and precisely controllable plasma segment can be maintained between the electrode and the point at which the plasma arc is intended to enter the tissue. As a result, the method becomes suitable for additional applications (e.g., in the area of neurosurgery and for lesions of the oesophageal mucosa) for which HF surgery could not previously be used because of the great risk of thermal damage to adjacent parts of the tissue.
Another substantial advantage lies in the fact that with the proposed solution the high-frequency energy is essentially all converted to heat at or within the tissue, because no effective resistances are required for control. This eliminates the need for large cooling surfaces or for active cooling by means of ventilators (which is extremely undesirable in the clinical context), and provides considerable savings with respect to the complexity and size of the device as well as the energy expenditure during use. The device thus becomes more acceptable for clinical applications.